Street illumination



1932- M. B'. BECK ET AL STREET ILLUMINATION O'riginal Filed July 18. 1951 8 Sheets-Sheet 2 ATTORNEY 1932- M. B. BECK ET AL STREET ILLUMINATION Original Filed July 18. 1931 FIG. 2

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A, mam) ATI'GRNEY I Dec. 13, 1932. y BECK ETAL Q U 1,891,137

STREET ILLUMINATION ori inal Filed July 18. 1931 s Sheets-Sheet 4 I III.

INVEN R Kilo ATTORNEY Dec. 13, 1932. M. a. BECK ETAL STREET ILLUMINATION Original Filed July 18. 1931 8 Sheets-Sheet .5

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' i l INVE TO ATTORNEY 1932- M. B. BECK ET AL STREET ILLUMINATION Original Filed July 18. 1931 8 Sheets-Sheet 6 Fl G. H.

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0 0 5 4 w z m Lifiux hrh bsmibw iRmN KO LEuNvWmQ 617/0 of jreffr Mar/l fa Nam) T/A/G 14/50/17 7 "W" 5% W 5. 4f ATTORNEY Dec. 13, 1932. B ET AL 1,891,137

STREET ILLUMINATION Original .Filed July 18, 1931 8 Sheets-Sheetv 7 4 I ma AT ORNEY Dec. 13, 1932. M 5, BE ETAL 1,891,137

STREET ILLUMINATION Original Filed July 18, 1931 8 Sheets-Sheet 8 Patented 13, 1932 UNITED STATES MORRIS B. BECK, or NEW YORK, AND Joint n. wHIrrrAKER, or BABYLON, NEW roux,-

ASSIGNORS T WELSBACH srEEEr LIGHTING COMPANY DELPHIA, PENNSYLVANIA, A conrosarron 0E DELAWARE PATENT Fri s STREET ILLUMINATION Substitute for application Serial No. 551,732, filed July 18, 1931.

. I Serial No. 636,240.

. This application is to take the place of application Serial No. 551,732, filed on July Our invention relates to efiicientplanned scientific street lighting. It is authoritatively recognized by illuminating engineers and those versed in the art of street-lighting, that the flux of light from any light source, since it radiates in all directions unless controlled by controlling and directing accessories is totally unsuited for the production of such prototype light-distribution curves as are desirable and necessary for that quality of substantially uniform horizontal illumination required in planned scientific street-lighting. It is also well understood that even when street lighting units are used which are properly designed to produce the required prototype light-distribution curves but do so symmetrically, that is, when the candlepower distribution is the same in all vertical planes about the source, a large percentage'of the light emitted downwardly by the unit reaches the ground outside the boundaries of the street pavement and sidewalks, which constitute the real working planes, and is either largely wasted or actually becomes an ob j ectionable annoyance to those owning and/or occupying the@ property adjacent to the street-lighting units.

Some illumination is required, and desired, Y

on the curbs and sidewalks and even a sufficient amount on the lawns, residences and other buildings for adequate police protection and other afety-functions of streetlighting, but beyond such requirements, all light which falls outside the working plane must be considered. inefiective. Authorities agree that since a street has a long narrow area to be illuminated, this lack of lateral control will in many cases, allow a considerable portion of the light to be'delivered outside of the street and sidewalk area. By directional control of light in lateral planes this condition can be remedied.

If, therefore, this objectionable and ineffective light can be so controlled and directed as to be prevented from falling into unde sired places and can be directed upon the actual working plane of the street, the efficiency of the street-lighting system will be greatly increased and its effectiveness largely enhanced. The following definition has been given:

- Asymmetric means not symmetrical. It means redirecting the light in side direc- 1310118 to deliver more toward the street than toward the houses and lawns.

It has been truly said: The waste of light on American streets due to lack of directional control is serious.

The proper application of light-controlling equipment'would increase the efi'ectiveness of street lighting in general by a large percentage. Such equipment, while widely used today, is not as widely used as conditions j ustify. Light is just as valuable as the electricity which produces it or-the dollars which buy the electricity, Wasting light is exactly the same as wasting electricity and wasting money. Good engineering in street lighting will take this factor into account.

Prior to our invention practically all attempts to accomplish the above object'employed the principle of light refraction and notlight-reflection and all made use of outside light-controlling and directing accessorles, such as prismatic glass refractors.

While the. desirability of the aforesaid asymmetric directional distribution of light for eflicient street-lighting has been admitted by illuminating engineers for years to be the ideal for which to strive, its practical adoption by the public to any considerable degree in street lighting in America has been greatly deterred and delayed by the large expense involvedbythe use of the necessary accessories and equipment to transform and reshape the bare-lamp distribution by means of retracting glassware, the only type able to accomplish this object before our invention. This excessive expense consists of three factors 1 increased investment, due to relatively hig cost of glassware and-accessories; (2) high maintenance cost, due to replacement of broken glassware and accessories, outer enclosing globes broken by falling pieces of glassware inside the luminaires, and the cleaning and adjusting of the equipment; and (3) the loss of efliciency, necessarily in- OF AMERICA, 0F PHILA- This application filed October4, 1982.

volved through the absorption of light refraction, resulting in relatively low overall efliciencies of such refracting units.

These were the conditions in the art which confronted us before we made our invention, the objects of which are to produce a process which if followed, eliminates or minimizes, the aforesaid objections, and through the medium of our asymmetric directional lightcontrol equipment, the objectionable excess light, which otherwise would fall on the abutting properties and buildings, is directed on to the sidewalks and pavements. A further object of our invention is to produce a process, which if followed, will permit the attainment of very much lower investment and maintenance cost.

A. further object is the attainment of very much higher efficiencies by entirely eliminating all outside reflecting and refracting accessories and their consequent expense and losses, and by utilizing the relatively large percentage of light which is distributed outside the street boundaries by all street-lighting units which have a symmetric distribution curve.

We have demonstrated in actual practice 1 that over all efliciencies, that is the ratio of the total light output of a street lamp constructed in accordance with our process, to

, the total lumens emitted by a bare unprocessed street lamp, as high as 110% can be attained in the transformation of the bare unprocessed lamp-distribution into the ideal.

A furtherobject is to attain the foregoing and yet, by following our process, produce an electric light bulb, useable in scientific street lighting and which is applicablevto all standard types of street-lighting equipments.

A further object of our invention is to provide a process which, if followed, will result in the production of an electric streetlighting bulb in which the adjustment of the light directing media, necessary to roduce the predetermined ideal light-distrlbution is immovable and' therefore permanent.

A further object of our invention is to produce a process which if followed, will result in simplified lower-cost maintenance of street-lighting systems, due to the elimination of the cleaningand adjusting of all aux- 1l1ary glassware and accessories'exterior to the electric street lighting bulb; eliminating of the handling, delivering and stocking of such relatively heavy and fragile and the saving of rentalfor, and maintenance of, the storage space, for such equipment.

A still further object of our invention is to produce a process, which if followed, will eliminate'the lack of permanency of the adjustment of auxiliary glassware and accessories, with the consequence that the reliability and quality of the illumination results are increased.

uipment, 1

A still further object of our invention is to produce a process, which, if followed in the construction and operation of planned scientific street-lighting systems, will realize the Formulae (1) When a is less than tan tan (1 1 sin cos" (2) When a is greater than tan (cp.)a=K a M (cp .)a=K

It is possible for one snfiiciently versed in mathematics by means of these formulas, to

ascertain by the accompanying speclfications,

how such curves are constructed, reference being made to the accompanying drawings' We shall proceed to describe the process by which we are able, as demonstrated in actual street lighting practice, to accomplish the objects herein set forth.

Referring specificallyto the drawings:

Figure 1 is a diagram of the symmetricideal pliototype curve for street lighting where =4;

Figure 2 is a diagram of the symmetric ideal prototype curve for street lighting where M =6; v

Figure 3 is a diagram of the symmetric ideal prototype curve for street lighting where M =8;

Figure 4 is a diagram showing the relation I of varying spacing distances to mounting heights for the values of M corresponding is a diagram showing the relation ing heights for the values of M correspondre 3; is a tabulation of prototype curve, candlepower values when various values of M are substituted in formulae (1) and (2) Figure 8 is a com arison of a light-distribution curve actua ly attained in practice from a symmetric street-lighting unit constructed in accordaiiciiwith our process with the light;distribution curve, delivered by the same gas-filled, tungsten-filament, incandescent, series street-lamp, before it was proc-. essed. In this instance the lamp is processed ing to Fi Figure ed onv these surface-sections to so as to deliver a small amount of lumens upwardly to be utilized in any desired manner, such as illuminating the upper portion of an outer enclosing globe, etc.

Figure 9 shows typical exploratory lightdistribution curves about an asymmetric directional street-lighting unit, such as are usually observed and plotted to determine the form .of the solid, or tore of light emitted by the unit and the candlepower values at the various angles about the unit.

Figure 10 is a comparison of the light-distribution curves of a bare lamp, a street lighting unit constructed in accordance with our process, and the ideal shape of prototype curve for M =6, together with a distribution curve from a street lighting unit actually constructed by an exponent of the handling of light by refraction as hereinbefore outlined. These are symmetric types.

All light-distribution curves shown herein give 0 to 180 readings only, in the customary manner.

Figure 11 illustrates the method of determining what proportion of the zonal lumens emitted by a street-lighting unit having a symmetrical light-distribution curve, will fall upon the working plane of the street and what proportion will fall elsewhere;"

Figure 12 is a graph of curves giving the percentages of zonal lumens from each zone of a street-lighting unit having a symmetrical light-distribution, which will be eflective on the working plane of the street at various ratios of street width to mounting height;

Figure 13 is a erspective diagram of an assymetric streetighting unit U and a section of a street illustrating the method of calculating the number of lumens intercepted by the various surface-sections of the street, and the additive and subtractive lumens needproduce the desired illumination;

Figure 14 is a comparison of the vertical light-distribution curve, A, of a clear 2,500 lumen lamp; the curve, B, of a clear 4,000 lumen lamp in a difiusing enclosing globe as ordinarily used in street-lighting; and lightdistribution curves, for m=8, of a streetlighting unit constructed out of the same 2,500 lumen clear lamp and the same difiusingglobe, in accordance with our process, these curves being taken in certain planes, namely, C, at right angles to the curb, or center line, of the street; D, at 22% degrees outwardly from the curb, and, E, approximately in the 87-degree maximum lateral cone;

Figure 15 is a diagram for the location of light rays emitted from a point source and showing in any difierential vertical angle formed by two planes intersecting along theaxis of the bulb;

Figure 16 is a diagram representing a sectional plane taken in said differential verti- I cal angle; and

the bul flux included wit-hm the prototype symmetric curve.

M: distance between adjacent light units.

height of light unit above street h, k, h"=various heights of the street-' lighting units.

D, D, D"=various spacing distances corresponding to the above mounting heights.

(Z, d, Z=various distances from base of post to mid-points between units.

U=street-lighting unit.

S=spherical angle which includes a zonal street surface-section under consideration.

H=the horizontal angle which locates the plane of the average. or central, ray for. a given zone under consideration in Figure .13.

V=the vertical angle which locates the central ray for a given zone under consideration in Figure 13.

V, the vertical angle to the midzone plane parallelto the curb in Figure 13 measured on the quadrant across the street.

V the vertical angle to the midzone plane perpendicular to the curb in Figure 13 measured on the quadrant parallel to the street.

Y=one-half the zonal angle inclosin the surface-section of the street under consi eration in Figure 13. For IO-degree zones, Y would be o-degrees. i

I='nten sity of illumination in foot-candue to the light that is reflected once from the spherical surface g, Figure 11, and emerges at angle it!) through the spherical portion of L ((1) represents the light intensity of processed lamp in that plane at angle (a) due to the light that is re ected once from the conical surface 2', Figure 11,and emerges at angle (a) through the spherical portion of the bulb;

I ((1) represents the light intensity of the processed lamp in that plane at angle (a) due to the light that is reflected once from the conical surface 2", Figure 11, and then once from the spherical surface g, Figure 11, and emqfges through the spherical portion of the bul I (a') represents the light intensity-of the processed lamp in that plane at angle (a due to the light that is reflected once from the conical surface '5', then from the conical surface 2', then from the lower spherical surface f, then from the conical surface 71, then from the conical surface z',-all in Figure 15, and emerges through the lower spherical portion of the bulbat an angle (a') which is not equal to the angle (a) a The parameters of the lamp and other symbols used areas follows:

On the left side of Fi ure 15, I

e is the angular sprea from the nadir, of a transparent portion of'thebottom of the spherical portion of the bulb;

f is the angular spread of the opaque specular reflective medium on the lower spherical ortion of the bulb, when e= 0, i. e., when sai portion is completely covered;

fe is is the angular spread of the opaque specular reflective medium on the lower spherical portion of the bulb, when 6 is greater than 0, i. e. when a transparent portion of the lower spherical part of the bulb exists.

2' is the angular spread of the opaque s ecular reflective medium on the conical portion of the bulb;

g is the angular spread of the opaque specular reflective medium on the upper spherical portion of the bulb;

9 is an angular spread of the opaque specular reflective medium on the upper spherical portion of the bulb which is greater t an O and less than 9;

T is the point of tangency of the conical option with the spherical portion of the u On the right side of Figure 15,

e is the angular spread, from the nadir, of a transparent portion of the. bottom of the spherical portion of the bulb;

f is the an lar spread, measured from the nadir, of t e opaque specular reflective medium on the lower spherical portion of the bulb when 6 0; and is the angular position of the up(per edge of such area when e is not ual to f'e' is the angular spread of the opaque specular reflective medium on the 'lower spherical portion of the bulb, when e is greater than 0, i. e. when a transparent ortion of the lower spherical part of the ulb exists.

1,se1,1s7

z" is the angular spread of the opaque specular reflective medium on the conical portion of the bulb;

g is the angular spread of the opaque specular reflective medium on the upper spherical portion of the bulb;

9' is an angular spread of the opaque specular reflective medium on the upper spherical portion of the bulb which is greater than 0 and less than g;

T is the point of tangency of the conical portion with the spherical portion of the bulb; and further;

r is the radius of the spherical portion of the bulb;

k is the vertical distance of the light source irolran the center of the spherical portion of the Angle (a) is the angle at which any ray of light emerges from the bulb, called alpha;

0 is the angular position of any ray of light in the axial plane, before it undergoes reflection; or the angular direction of any barelamp ray, both measured counter clockwise from the nadir; v

s is the angle between the axis of the bulb and the sides of the conical portion of the bulb, measured from the nadir;

n is any integer, designating a certain light-ray;

m is the number of times a particular lightray is reflected R represents the reflection coeflicient of the opaque specular reflective medium;

do represents any small, or differential, vertical angle formed by two planes intersecting along the axis of the bulb and in which the given mathematical equations are solved;

JJ represents a sectional plane taken in said differential vertical angle;

QH is a line drawn perpendicular of ray WZ, or L (a) v V is the vertex of angle a, between the conical sides extended to intersect the axis;

represents the an le between line HW and the axis of-the bull);

H represents the center of the spherical portion of the bulb;

S represents the location of the center of the light source.

In planning practical street lighting systems utilizing our invention, it is only necessary to follow the procedure now given in our specifications. If we assume that substantially uniform horizontal illumination is the result desired and assuming, also, the value of M= 6, we may calculate and construct by means of the prototype curve-values given in Figure 7. an ideal prototype curve for symmetric distribution.

Havlng constructed such .a prototype curve l of li ht-distribution, we now have a pattern to gu1de us in the design of the reflecting surface or surfaces, to be-placed upon' our eleclll till

earner trio light hulh and their matude and location, or placement. Since the light emanating from the usual forms of electric light hulhs, when in its natural state, as far as the shape of its curve of distrihution is concerned, is very poorly adapted to the production of eldective, practical illumination of almost any specific work space and especially for the ideal uniform horizontal illumination of the streets, boulevards, highways, and other thoroughiares, as Well as airports and other large outdoor public spaces, where planned scientiiic illumination is desired, it is necessary to so alter and remold the natural tore, or solid of light emitted by the hare street lamp as shown in curve A n igure 10 as to reshape it into practical compliance with the shape indicated by the ideal prototype curve B in Figure l Fortunately it is Within the ahility of practical illuminating engineers to accomplish this purpose providing a process is at hand, because the crude mass of light sent out hy the usual form of electric light hulhs is a very plastic medium, and each and every ray of it can, lay such proper procedure, he easily hent hy reflection and redirected into useful planes and the vvhole light-mass, molded into an ideal form for the solution of a given prohlern, such as planned scientific street lighting, providing a process and the means for the transformation of the natural nonuseilul light-mass from the hare electric light hulhs can he provided. lt is with such a process and the concrete structure which will produce the resultant transformation in an efiective, eficient and useful manner that our invention is concerned.

ln Figure 1d, all curves of vvhich are symmetric:

Curve A represents light-distribution from hare street-lightinglamp.

Curve B represents the shape oi the li htdistribution from prototype curve foot drawn to scale) Curve (3' represents light-distribution from a street-lighting unit constructed in accordance with our process.

Curve D represents light-distrihution from a street-lighting unit equipped with prismatic reiractor.

We may now proceed with the transforma tion of the hare-lamp curve into the prototype hy determining the additive and subtractive values of candlepovver at all angles to reshape it for practical use, for smetric distrihution.

from curve A (Figure 10) =hare lamp cp. directed at angle a.

00 from curve B (Figure 10) =required prototype op. at angle a.

he (Figure l0)=cp. required to he added to 05 to produce 00.

Therefore bc=0c-0b=cp. requirfl to he added to 0b to produce 00; use can he do directly of the values in Figure 7.

By repeating the above process for each degrees, starting with 5 from the nadir, the required additive and subtractive candlepower needed at each angle can be ascer- Multiplying factors to obtain some lemons from average m ccadlepower Multiply- 0 to 10 170 a 180 0.095 m m an we to 110 uses 20 to 150 w 1a)" one so to or 1 0 to 150 uses 1'10 130 $0 1 9 0. 774 59 t0 60 20 to 130 0.897 60 m we 110 a 120 can to em in 1.05s so to an 90 :0 100 1.091

'When extreme accuracy, or accuracy greater than that given by the alcove choice of 10 degree zones and their constants is desired, zonal angles of any desired magnitude may he chosen and inlike manner their constants determined and used.

The lll de ree zone chosen herein is the one made use of in all practical Work of this char actor in illuminating cheering.

To use these factors with the curve of any lighting unit, the candlepower at 5 degrees is multiplied by the 0 to 10 degree factor to oh tain lumens in the 0 to 10 degree zone; the candlepoiver at 15 degrees is multiplied hy the 10 to 20 degree zone factor to obtain the lumens in the 10 to 20 degree zone, etc. The zonal lumens for any large zone is the sum of: the lumens thus determined in all of the ill degree sections oi the zone.

Having thus determined the deficiencies of the hare lamp-distribution in zonal lumens for eachof the 10 degree zones as ahove outlined and having determined the required ad= ditive and subtractive lum'ens needed in each zone, we then spread the specular reflective substance over such predetermined areas on till will

lid

the surface of the light bulb itself to supply the already ascertained deficiencies in each zone. We have found in practice that when M=6 the top line of the reflector-substance surface on the loulh subtends an angle offill the unit will fall on the street.

degrees at the center of the filament and the bottom line of the upper zonal reflector substance subtends an angle of 170 degrees at the center of the filament, when M 6 and the bulb is mounted base up.

We have also found that the top line of the reflector substance of the lower zonal reflector surface subtends an angle of degrees at the center of the filament.

Inasmuch as the location of the center of the filament in manufactured lamps will not uniformly occupy the center position of the bulb, it is obvious that the exact position of these zone-boundaries will vary accordingly.

Having thus described the process of forming a reflector on an electric light bulb for symmetric light-distribution suitable for obtaining substantially uniform horizontal street illumination without any controlling or directing accessories, we may now proceed with the process of transforming this symmetrical light-distribution into an asymmetric directional light-distribution suitable for the utilization of the zonal lumens which in said symmetric light-distribution, fall inefiectively outside the boundaries of the street pavement and sidewalks.

Either the point-by-point method, or the zonal-flux method, may be used for the determination of the number of lumens which will fall on the Working plane of the street and the number of lumens which will fall on either side of the working plane of the street.

For the light-units which have a symmetrical distribution of light about a vertical axis, the zonal-flux method of making these determinations is far less laborious than the point-by-point method.

The zonal-flux method is based on the fact that, with any given unit which has a symmetrical distribution of light, only a certain proportion of the total lumens given off by This proportion depends on the street width and the location, mounting height, and vertical light distribution of the unit, as sketched in Figure 11. The curves in Figure 12 have been worked out in the literature of the art and show the percentage of the total lumens, in

each 10 degree zone, which will fall on the working plane of the street, for any given ratio of street width to mounting height of the lighting-unit. From this data and from the symmetrical distribution curve of the unit,- which gives the lumens emitted in the various 10 degree zones, the total number of lumens falling on the working plane of the street and oiii, may be calculated.

other words, the question can be determined as to how many of the total lumens emitted by a street-lighting unit are effective on the working plane of the street. It is borne in mind that the primary object of street-lighting is to illuminate the street pavement, secondarily the sidewalk areas and incidentally the lawns, building-fronts, etc.

We then find by subtraction, the number of zonal lumens in each 10 degree zone which would fall in ineffective locations, the summation of which ineffective lumens are available for redirection by reflection into useful zones, less the small loss caused by such refiection.

Because of the asymmmetry of the light distribution from street-lighting units using specular reflective surfaces on the bulb to produce asymmetric directional control of the light, the above or any similar zonal-flux method of calculating the distribution of light on the streets working plane cannot be used, since no single value of candlepower can be assigned to any zone and be called the average for that zone.

A complete photometric exploration of such a street-lighting unit would consist not only of one vertical distribution curve, as is the case with a symmetric unit, but of a series of them showing the horizontal and vertical distribution curves in planes at various angles to the axis of the unit and the accuracy of the method is increased as desired by in creasing the number of exploratory curves ad libitum.

Figure 9 shows the general shapes of exploratory curves of this nature taken at various angles about a street-lighting unit. Thi particular series includes a vertical distribution at right angles to the curb, a vertical distribution in a plane at 10 degrees out from the curb through the maximum zone, and a dis tribution taken at a vertical angle of degrees from the nadir, which is the maximum lateral angular cone for this particular unit. At this angle the asymmetric directional nature of the distribution is most pronounced, the candlepower reaching a value of several times the rated candlepower of the clear lamp alone. Similar curves can be taken at various other angles and planes to secure the required data to solve a particular problem.

A method of determining the lumens in each, and/or all, zones, efiective on each, and /or all, unit surface-sections of the working plane of the street as defined above, has been developed for an asymmetric directional street-lighting unit, as follows:

A. diagrammatic section of a street with an asymmetric street-lighting unit mounted, at U, on a lamp-post of any height is shown in perspective in Figure 13.

lln the vertical plane passing through the unit and directly across the street at right angles to the curb, an arc of QO-degrees is drawn and another one is drawn in a plane through the line of the street lights. Each of these quadrant-arcs is divided into desired zones as indicated and planes are passed reenter through to the points of division on the quadrants. 'lhe planes thus. passed through the quadrants will be at right angles to each other and their intersections with the street surface will be lines forming rectangular sections of the, streets surface, such as that indicated portion or portions of the street desired to be surveyed.

lhe value of the spherical angles S can be found either by the standard method of double integration for an area, or by the approximate trigonometric formula:

by the intersection of the and degree planes intersecting parallel to the curb and the and degree planes intersectingnt right angles to the curb line.

The number of lunlens emitted by the one light-source U, and directed by it into each surface-section similar to the one just de-v scribed, can be calculated by means of theprinciple involved in the definition ofthe lumen, since a lumen (the unit of luminous flux) is defined as the flux emitted in a unit solid angle (steradian) a source of one candlepovver. Therefore the lumens directed through each of the pyramids formed by the above described intersecting planes can be obtained by multiplying the average candlcpovver of the source over the pyramid in question spherical angle, S. The average candlepovver is talren as that in the direction of the ray joining the light source, U, and the middle of each street surface-section and it is determined by the horizontal angle and the vertical angle V. Simple tri'gond metric derivations give the following equal tions for obtaining the values of these anglee 2 V= (tan l/i tan V3 72 tan l7,

tanH= l ables of the values of these angles have been prepared and are available in the literature of the art or their values can be calculated for each case under investigation. Having obtained them it is now possible to read off A table of these values of candlepower, as

thus calculated for the street-lighting unit 7 under investigation, may then be compiled- Such tables as those described above, usually include for each zonal section, values of horizontal angle, H; vertical angle, V; sphericalangle, S; candlepower 0p. at the center of S; the number of lumens, lb, falling upon the street surface-sections through each spherical angle, S,and strip totals as well as grand totals of lumens falling on the street, or any (1 an v.+ an va when Y is half the spherical angle of which V, and V, are the midzone angles. As explained above, the lumens in each of these spherical angles can now be obtained by multiplication of the value of the spherical angle.

areas, or for the total area, by dividing the 1 total lumens-value for a given area by the area in square feet of the surfacesection under consideration.

By the use of the above method the addi tive and subtractive numbers of zonal lumens needed to mahe'the necessary changes in the light-distribution of the street-lightingunit under investigation to transform the light distribution of our symmetric processed elemtric street-lighting units into the asymmetric directional type of light-distribution required for our eficient process of producing street-lighting from asymmetric lighting units, by so spreading opaque specular rellective substance over predetermined areas of the surface of the electric street-lighting bulb as to redirect the required additive and subtractive zonal lumens into the proper zones, or surface-section areas, as may be required.

The determination of the candlepower required to be emitted at any angle by the street-lighting unit having an asymmetric directional light-distribution curve in order to produce the desired illumination on the Working plane of the street, or the desired intensity at any and/or all, points, is made by the following Well known formula:

c It

cos o ny the use of this formula, a complete analysis of the candlepower distribution necessary to be emitted by our asymmetric streetlighting unit to produce the desired illumination on the street, can be made.

Having thus determined the deficiencies and excesses of the bare-lamp distribution in both candlepower and in zonal lumens, as above outlined, their control by opaopie specu- Mill its

lar reflecting areas on such portions of the surface of the bulb as will intercept some of or all the rays of light in directions in which the bare-lamp light-distribution curve exceeds the prototype light-distribution curve, will so redirect said light by one or more reflections that said light emerges from the bulb in directions in which the prototypelight-distribution curve exceeds the barelamp distribution curve, thus supplying substantially all of the candlepower and lumen deficiencies of the bare-lamp. The following mathematical equations express the relations between the said bare-lamp and prototype light intensities and the lamp parameters and constants, for any difi'erential vertical angle formed by two planes intersecting along the axis of the bulb; and when these equations are solved for the particular conditions, the ideal to be attained is established.

P 0 I1 1 (a) I (a) (a) a for values of angle (a) greater than v O --1 120 tan 1+ k and Bax than 0 -1 sin 79) 120 +29 tan (60o+g) 1 (1b) and also less than and (a) =0 for all other values of angle (a) and wherein angles a and 6 are related by the equation with {greater than 180 but less than 860; and where a=6+2 sin sin also,

a in 6129 12 1 for all values of angle (a) greater than 120 tan and less than 120 tan +Sin g 1 cos 9 and also less than g tan 7) 60 tan' la 1 2% r 13 a sin 0 sinada for all values of angle (a) which result from the equations when the parametric angle 9 is greater than zero and less than 9 and the parametric angle f is either greater than e and less than c or else greater than f and less than (1209 g) and (a) =0 for all other values of angle (a) and where angles (a) and 6 are related by the parametric equations (5) a= 180 4 tan- LLti LQQ with dgreater than 0 and less than 180.

A high degree of accuracy or close approximation to the prototype, if desired, can be obtained by means of this disclosure for any number of values of light intensities at all selected angles; and in all differential vertical angles. lVe will now Work out typical application to candlepower intensities to exemplify the action of the reflecting areas.

Suppose now a beam leaves a light source in a sphere at angle 6 with the vertical, having an intensity 1(6). This light has symmetry about a vertical axis. lts solid angle can therefore be taken as a zone. Let 010 be the angle subtended by the zone, then its solid angle is L 2X3.1416Tr sm 0 d0=2x3l416 Sin 6 Suppose that by reflection, or reflections,

- Learner llts spread, do, ma be difierent now. Any- Way, the solid ang e subtended at the appar- 5 ent source is a xsnue-r sin ads thus the beam started ed with an intensity 1 (6) in a solid angle W=2xaisre star do After reflection it is confined to a solid angle 2X3.l4ll6 r sin a do 7 =2 3.l4losinade.

its intensity is now I Tithe solid angle aX 3.1416 sin e do is larger than 2 3ldl6 sin 0 aid, the intensity is reduchd by the factor I 7 7 sin 6 dd sin a This is what the foregoing expression says,

. When a beam is reflected lay a plane or conical suri ace,

does not equal 1. I a

Where em a is greater than sin 6, and ole is greater than 9,

sin a dd sin a. do

is less than 1, and the heam is reduced in loy reflection We have treated the source of light as a point source only. Any actual filament will have a center of Brightness at some point, equivalent to a point source, The chief practical difierence between the actions of a oint source and an actual source is that the limiting edges of beams of light will not be sharp in the latter case-they will he rounded 0d and this is a good thing. i g i vertical dis- We consider the source at a ,tance it from the centeroi the spherical portion of the bulb. This distance 72 is one of the lamp parameters. The others are listed and defined above.

For the purposes of illustration, we will now proceed to show how the values of the.

various intensities in any direction were oh tained. The bare lamp curve is changed hy reflections at the opaque s ocular reflective surfaces At any angle fa) the intensity will he that or the harelamp plus the brought about loy reflections-single reflec tions, douhle reflections, and others We have treated each gain separately and have labeled them K1 I2 (a) I3 (61} a c a Ifld q T m) is the gain at angle (e) due to a ray which undergoes a single reflection from the spherical reflecting area g or as shown in Figure 15, angle g" denotes angular spneedot the spherical area that is coated on the ardal plane selected, Angle gvis the quantity that limits angle (gig and the range of il (a);

if angle g is me e greater the range of T (at) is increased, Thus, in Figure 1d, ii 'angle e? is increased so that the reflected ray strikes the hottom edge of the spherical portion of the hulh, any further increase in angle e de creases the ran e of 31 w), The other oi the range is dried hy the point of tangency of sphere and cone, T or T.

it will readily he seen from. theahove dis cussion that if the angular spread of opaque specular reflective medium he extended to the difierent limiting points on the right hand side of the amis oi the hulh than on the left hand side, ic e, if the values of e f 9 and c are not identical with e f g; in all amial planes, hut are diderent in every ardal plane or in different groups or? planes for difierent reflective areas, then the lamp will give a light-distribution which is not metrical around the central axis 'Vlil oi the lamp hut is ;r n :netrical around such axis and the lamp, when lighted pro duce asymetrical uniiorm, homontd street illumination. For such a lamp, there fore, it is necessary solve for every value or group or values, of each or? the 1;uentities efi, g, and i and e 9 g" and t". We have now completely analysed the etlect oi the location of the areas of opaque specular r dective medium iorsuch asymmetrical li'glmt distribution "We have derived a mathematical relation for lids), to express its intensity at any llii "ice

their primes, and so on. By changing e f y g, t, and so on, we can change theintensity in any direction, i. e. by arranging the limits of the areas of specfilar reflective medium we canchange the intensity in any direction direction of angle (a), for this mathematical relations, give the 1(a) values 7 whose summation will approximate the I (a) values of the prototype light-distribution curve.

The bare-lamp has a certain intensity curve. We may denote the intensity of its light ray at any angle 6 by 114(6). 8 is reckoned from a downward vertical line, or nadir, andcounter clockwise.

I, (a), I (a), 1,,(a) will be written in terms of L10).

99 is the angle that the original bare-lamp 2o ray makes with the vertical, or nadir, before reflection but after reflection (or reflections) it makes angle (a) with the nadir.

We determine an expression for 0 in terms of angle (a) in every case. Then for a given angle (a), 0 is found, and 1,,(6) can be read from the bare-lamp curve.

Also

sin 0 d0 d cos 0 sin a (is r d cos a m can be calculated. In this way e R I: (in,

can be calculated, and this is the gain in the particular reflection (or multiple reflections). Y

The summation of bare-lamp candlepower at angle ag. and the gains in candlepower m at angle a brought about by the intensifi la), Lewis) na) p duces the approxlmatlonto the prototype mtensity I o:-) at angle ((1). Therefore I (a)=I (a) +l (a) +I (a)+ 4; In) M) The following is the investigation and determination of 1 0:). 1 is the sin in intensity at angle (a) caused by one re ection from the upper spherical portion g of the bulb, Figure 15.

' As shown in said figure 15,

=ic sin angle YSI-E=sin 130) sin a cos angle cos (6- 189") cea t m angle HWZ angle YWH= sin' g a=9+2 angle HWZ- 180 y (1) a' a+2 sin 9 180 (aterms of 6.)

Angle a is a minimum when the ray 1;, (0) strikes the point of tangency T Then =60 and 01s0=mnfr 2 tan 1+2]: l r 2 r and I a =2 .60tan* -di 120 tan +21: (1a) Angle a is maximum when the ray I (0) strikes the bottom of g The maximum value of angle (a) may also be limited by the angle g, i. e., the coating on righthand side.

' Suppose the point of impingenceW of the ray is not at the bottom of the angle 9 on the left side, but, that point Z of emergence of L (a) from the bulb is at the bottom of angle g on the right side. That is, angle 9 may be coated below point W so that this point W is not the limiting factor; rather, the point Z of emergence determines the maximum value of angle (a), If angle (a) were any greater the ray would strike the. coating on angle 9.

The relations are then angle WHV= angle WHY+ angle Wsn=sn (e- 180) angle VHQ= 82 angle z= were 2 angle wnv+ angle VHQ= sin" 4- angle VHZ angle VHQ angle QHZ also when Z coincides with lower limit of 9", angle VHZ 60 9 therefore -lc sin t This is the maximum value of angle (a) when the ray is cut ed by g".

In other words 1 M) lies say hetween two values of angle (a) Une limit is fixed by the point of tangency 'l of sphere and cone. The other limit depends on angle 9, so by varying g the range of LM) can be varied. Angle 9' denotes the angular spread of the upper spherical coated area. lit 9 is increased the latter limit is increased, unless the ray SWZ strikes the bottom of the zone at g. When this happens, any further increase in y decreases the range of l (a) in this direction,

sin 6 sin a can he found. Now we get d9 2k i. cosa and are

sine sina In practice,

is small, so

can he neglected in comparison with 1. With time approximation,

sin

(when the angle is measured in radians). 'lhen ta) =a at) 3g 2 =a he ieose 2 We can now solve this equation for Ltd), which can be done, since they are expressive of the light intensity due to the location of the opaque specular reflective medium on the bulb surface, and all the quantities on the hecornes 12W "KL/g ten 27:

We also find that the upper limiting, or maximum, value of angle (a) and l w) ray when it strikes the hottom of the opaque specular reflective medium on the emergence side is c= lac atel+cos g't and that for e limiting value of angle ((2) Mill ' lltl when the 1 ray strikes higher than the upper edge of the cone,

a= 60 tar- 2+ tan 6 and 0 (1.=2 18O0-- a=300-0 9=300a v For I (a) these relations exist:

I: 180 tan z+sm 4 o r 3 *X 180 -6 =---o- 3 7 180 cos 3 20 4 tani+ 0= 180tan' kv a=240um- 2 amp f) .'.a=300-(240tan r tani+ a.=60+tan' as above 1+ 8 4 2? sin a=sin 300 cos 0cos 300 sin 0 sin: ficos8sm0= (sm0 2 1/ 0 Alsod8-da L0 =R n i {-3 cot a) R r. f' /3 cot a), which can now be solved.

In the foregoing discussion of I (a) it was assumed that the opaque specular reflective medium is spread high enough so that the top of the medium on the cone is not the limiting factor, but that the bottom of the zone on the emergence side is the limiting factor. But if the cone is not coated sufficiently high, the cone itself provides the limit.

So, also, the rays, may be traced and calculated which travel fromv the source to the conical surface, thence to the opposite conical surface, and thence emerge through the 65 spherical portion of the bulb.

. to be reflected at the conical surface.

The simplest way of handling reflections from a cone is to put,a phantom source on the other side of the conical surface and imagine the rays, after reflection, to come from the phantom source.

Rays leave the source S at various angles, After reflection these rays are exactly as if they came from the image of S formed by the cone.

After the first reflection, the rays may strike the cone on the other side. After the second reflection, the rays act exactly as if they had come from a secondary image of the first, or primary, image of S.

Thus it is easy-to trace the rays in an axial plane, when dealing with the reflections at the surface of the cone.

As is shown in Figure 11, by the ray marked I (a'), this ray may be prevented from emergence after the said second reflection by placing opaque specular reflective medium on the bottom of the spherical portion of the bulb, as represented by f, of Figure 16, and be again reflected to the cone for two more reflections before emergence in the direction I (a'). As this direction does not contribute to the rays at angle (a) of our previous calculations, its effect will be only 1n relation to the prototype curve at the angle where it actually contributes.

By means of the equations and relations which have been developed and given above and other equations which have been or-can be derived in a similar manner, the path of any ray in an axial section of the bulb may be found for any given distribution of reflective medium upon the bulb, in any axial plane, the final angle of'emergence may be determined, and the contribution to the total intensity in this direction, angle (a) may be found by evaluating the expression:

which has been illustrated above.

For any given distribution of reflective medium on this axial section of the bulb, certain groups of rays will suffer the same reflections in the same sequence. For any one of these groups of rays 1(a) will be a continuous function. The boundaries of this interval maybe determined by substituting into the proper formulas, the angles, or lengths, giving the positions of the edges of the coated portions,which edges limit this particular group of rays.

For any given distribution of reflective medium it will usually be found that a certain few of these groups of rays are by far the most important and that if 1(a) is evaluated for these few groups then the light-distribution is determined closely enough for practical puriioses. There are, of course, a great many ifierent possible combinations 

